There’s vowing to exercise in the new year, there’s actually doing it, and then there’s actually doing it right.

For most of us, simple strategies and behavior modifications can help optimize our gym efforts. We should also keep in mind a few golden rules, such as not overdoing it at meal time and remembering that just sticking to cardio isn’t enough.

But there are so many ways new and lapsed exercisers commonly sabotage their workouts. Below, we offer advice on how to create a sustainable routine that will help you reach your fitness goals.

You don’t account for life’s curveballs

In the new year, people tend to shoot too high, says Danny King, director of performance and recovery for gym chain Life Time.

“Your goals should take into account the days when your workday blows up, you didn’t sleep well or it’s minus 10 degrees outside.”

King suggests having backup plans to slot in on days when you can’t get to your planned exercise routine. Try a fast-paced walk in your neighborhood or jog or run up and down stairs to raise your heart rate.

You’re wearing the wrong sneakers

Exercising in a worn-out pair of sneakers can lead to injury, because the shoes no longer have proper support. And if you already have balance issues, avoid maximalist shoes with chunky midsoles that can make balance even more challenging.

Ratty gym gear works fine, but King says investing in fitness apparel can give new exercisers a sense of belonging and confidence.

You’re lifting the same weight load

If you aim to get stronger, you need to progressively lift heavier weights or perform more repetitions, says Andrew Jagim, director of sports-medicine research at Mayo Clinic Health System in La Crosse, Wis.

Most people underestimate how strong they are. “I watch clients pick up their 50-pound kid and then tell me they can’t lift more than 20-pound dumbbells,” King says.

If you have been doing a chest press with 15-pound dumbbells for six months, you have probably plateaued. Try using 20 pounds and do as many reps as you can with good form, he suggests. Keep at it until it becomes as easy as the 15-pound load.

You think every workout must be hard

People like to go all-in, particularly in January. But jumping from zero to 100 often leads to burnout.

“I think the most harmful statement over the last 20 years is, ‘No pain, no gain,’” says ​​Mathias Sorensen, an exercise physiologist at the University of California, San Francisco. “You should not go hard every time at the gym, just like you shouldn’t only operate your car in fifth gear.”

He suggests new exercisers push themselves hard, or to the point they are exhausted, once or twice a week in their first year of consistent training. If you are pressed for time, make that time-crunched day count by making it your tough day and running uphill for five minutes. And don’t underestimate the value of recovery days.

You’re obsessing over the small stuff

If you are new to strength training, you don’t need to devote an entire workout to your triceps or abs, says Albert Matheny, a certified strength and conditioning specialist and co-founder of New York City’s SoHo Strength Lab.

“The majority of the population needs to focus on building a foundation and strengthening their bigger muscles,” he says.

Matheny recommends compound exercises that strengthen more than one muscle group at a time. Work the lower body with movements where you bend at the hip and knee, such as squat or deadlift, and the upper body with push and pull movements. A bent-over row targets the biceps, lats and shoulder muscles while engaging the core.

You’re using cardio machines without resistance

A fast, sweaty session on the elliptical machine might feel like a killer workout, but using zero resistance won’t make your muscles and joints stronger and can even lead to injury, says Adaeze Merenini, a New York City-based personal trainer with Crunch Fitness.

“Start with low or moderate resistance to develop strength as you get your cardio,” she says. And if something doesn’t feel right, check with a staff member to make sure you are positioned correctly on the machine.

You think cardio can replace lower-body strength

Merenini often sees runners and cyclists only working their upper bodies in the weight rooms.

“This can put excessive stress on the back, chest and arms, which can lead to muscle imbalances, lower-back pain and even sciatica,” she warns. “It’s important to also work your lower body and core to create a solid foundation to support the mechanics of running and cycling.”

You aren’t eating enough

Resolutions of improved fitness often pair with goals of healthier eating. For many, that means eating less. But if you are just restarting a workout routine, your body needs calories, especially after exercise, Matheny says.

“Your blood sugar drops during a workout, and if you don’t eat after, it continues to drop,” he says.

Even if you don’t feel hungry, he recommends a snack with some protein and carbs post-workout. New exercisers also need more protein to help with muscle recovery, he says.

You only use machines

Strength machines are like training wheels. They are a safe introduction to the mechanics of weight training. But they train targeted muscle groups and neglect your stabilizing muscles, Merenini says.

“Using free weights to perform compound movements will recruit more muscles and burn more calories,” she says. “And you won’t waste time waiting for a machine.”

You hit the gym without a plan

Think of gym time like your workday and create a to-do list.

Matheny recommends leaving your phone in the locker room to avoid the temptation of checking your emails and scrolling on social media. If you use your phone to stream your workout, turn off your notifications.

Your focus should be on your form and your breathing, he says.

Sign up for the WSJ Workout Challenge to boost your fitness.

Write to Jen Murphy at